A direct transition from a solid phase to a gaseous phase. Heated interplanetary dust or a comet can lose material through this process, changing its size and radiation-pressure coefficient or destroying it before gravitational or drag-driven evolution is complete.
For a locally linear ice temperature with positive gradient into the solid, a corrugated sublimation front receives a perturbation in heat flux proportional to its horizontal wavenumber . The Gibbs-Thomson effect suppresses short waves, giving the displayed quasi-static growth rate. It requires and , as well as a perturbation depth short enough for the linear background approximation. Unstable modes have .
For a sublimating surface with exponentially absorbed volumetric heating, use a frame moving into the solid at speed . Its steady heat equation is . The surface Stefan condition is . Integrating over the half-space gives the displayed energy-limited speed. The surface gradient is positive, but the far field is colder, so the temperature has a subsurface maximum where melting can begin.
An irradiated grain loses solid material by sublimation. Its lifetime depends strongly on temperature and composition. In a planetary dust tail, a short lifetime can truncate the distribution well before a relative orbital wrap.
A spherical black grain of radius absorbs luminosity through projected area and emits over . Radiative balance at sublimation temperature gives . Grain size cancels only under the stated unit-efficiency approximation; shielding and wavelength-dependent emissivity change the result.
At fixed irradiation, heating per grain is independent of temperature while cooling is proportional to . With positive heat capacity and intact grains, a positive temperature perturbation increases net cooling, and a negative one increases net heating. Fixed dust properties and sufficient dust-gas thermal coupling are necessary assumptions.
